On Spectrally Efficient Continuous Phase Modulation

Licentiatavhandling, 2000

Spectrally efficient partial response continuous phase modulation (CPM)
for wireless links is considered. Continuous phase modulation defines constant
envelope phase codes, which are advantageous when using non-linear amplifiers.
The channel is an additive white Gaussian noise (AWGN) channel and the
spectrum requirement is a mask, which puts stringent requirements on the
spectral main lobe and the spectral side lobes.
The thesis consist of three papers. In the first paper, a simplified
receiver is evaluated for high-complexity partial response CPM on the AWGN
channel. The simplified receiver consists of an approximate filter bank,
using simple integrate-and-dump filters, and a reduced state sequence detector
(RSSD). The complexity reduction is significant with a very small degradation
for the AWGN channel, with and without co-channel interference, compared
to the optimum maximum likelihood (ML) detector. The simplified receiver
is more sensitive to adjacent channel interference, than the maximum likelihood
(ML) detector, but a large complexity reduction is still possible with
a small performance degradation.
In the second paper, convolutionally encoded partial response CPM is
evaluated. A search for serially concatenated convolutional codes has been
performed. The somewhat surprising result is that no coding gain is obtained
compared to the best uncoded CPM schemes.
In the last paper, a numerical optimization of the minimum Euclidean
distance with the spectrum mask as constraint and the phase pulse shape
and modulation index as free parameters, has been performed. A small gain
in minimum Euclidean distance and an interesting property for some optimized
phase pulses, that could be useful for simplified receivers, were obtained.
There seems to be a saturation effect in CPM when using long phase pulses,
thus restricting the possibilities to define more powerful constant envelope
phase codes within CPM.
In addition, complementary simulation results for the 'best' optimized
CPM scheme, taking complexity into account, are provided in an appendix.
The advantageous property for the optimized phase pulse over a standard
phase pulse, when using a simplified receiver, is demonstrated.